Comparative assessment of physicochemical and functional properties of chestnut protein obtained by different extraction methods

Protein is the second most abundant component in chestnut dry matters. In this study, chestnut proteins were extracted with three different methods, i.e., alkaline extraction-isoelectric precipitation (AE-IP, pH 8.0, 9.0, and 10.0), alkaline extraction-ammonium sulfate (AS) precipitation (AE-ASP, AS saturation rate 30%, 65%, and 100%), and salt extraction (SE, NaCl concentration 0.5 M, 1.0 M, and 1.5 M), to understand how extraction conditions affect the yield and properties of chestnut protein preparations. Physicochemical (composition, color, particle size, surface hydrophobicity) and functional properties (solubility, water/oil holding, emulsifying, foaming capacities), along with the in vitro digestibility of samples exhibited significant variation across the extraction methods. For AE-IP and SE methods, protein yields ranged from 35.1% to 43.5%, while the AE-ASP method yielded between 13.2% and 49.8%, largely depending on the (NH 4 ) 2 SO 4 concentration. Purity of protein preparations varied between 82.4 and 93.3%, with the samples extracted with the SE method associated with higher levels of phenolic impurities. Ultra-centrifugal analysis revealed that for the native chestnut proteins extracted with all AE-IP and SE conditions as well as with 100% (NH 4 ) 2 SO 4 , the fraction of 11S was the dominant component, which was barely obtained with lower (NH 4 ) 2 SO 4 concentrations. In terms of functional properties, proteins from AE-IP method exhibited relatively higher water/oil holding and emulsifying capacities, while SE contributed to higher protein solubility but lower in vitro digestibility. Proteins extracted with 100% saturated (NH 4 ) 2 SO 4 concentration showed comparable functional properties to AE-IP proteins. This work provides theoretical insights into the potential use of chestnut protein as a novel plant-based protein ingredient.